This invention relates to a network system, and in particular a network system comprising a plurality of LANs (Local Area Networks) using hierarchical routing.
As one of the routing methods in a network system, in which a plurality of LANs are connected with each other, there is a known the hierarchical routing method on which standardization is based according to IEE 802.1. According to a description in IEEE 802.85*1, An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN (1985), .this is a routing method,, by, which relay control is effected by having a bridge mutually connecting two LANs uses a routing table indicating to which of the 2 connected LANs the terminal in an LAN connecting system is directed.
The routing table of each bridge is updated every time the relevant bridge relays a communication frame transmitted by the terminal so that the content of the routing table becomes more and more substantial. Suppose now, for example, that a bridge connecting a first LAN and a second LAN has received a frame having a source address (SA)="0001" from the first LAN. This bridge knows that there exists a terminal, whose address equals "0001" on the first LAN side, and registers the address "0001" of the terminal in the routing table, making it correspond to the first LAN.
Thereafter, when the fine bridge has received a frame having a destination address (DA)="0001" from the second LAN, the bridge refers to the routing table to be able to judge whether the received frame should, be relayed to the first LAN. Since each of the bridges registers the relation between the source address of a frame and the LAN, from the side from which the frame is inputted, in the routing table, every time a frame is received, if the address has not yet been registered, the content of the routing table is updated and becomes more substantial.
For executing the hierarchical routing described above it is presumed that the form of connecting the LANs in the network with each other has a tree structure, i.e. no loop is formed by several LANs at any part of the network. This is because, if several LANs form a loop with each other, there can exist plural paths between, two terminals, which gives rise to a problem that a plurality of the same frames are produced in the network. However, if the network is so constructed originally that the form of connecting the LANs with each other is a tree structure so as to avoid this problem other problems are produced in that there are no backup paths, if some hindrances occur, and there are restrictions to extending the system, etc.
Therefore the hierarchical routing system is supported by an algorithm to logically adopt a form of connection having a tree structure, starting from an arbitrary form of connecting LANs with each other. This algorithm is called a "spanning tree algorithm" and a constructed "tree consisting of LANs" is called a "spanning tree".
The spanning tree algorithm is carried out by sending and receiving a special control message called a "Hello message" between different bridges. At the initial setting, each bridge effects a broadcast transmission of a Hello message to all the other bridges. Each of the bridges has a bridge ID (identification No.), which is unique within the network, and the ID of the destination bridge and the bridge ID of a bridge which is the transmitter of the message, is written in the root identifier field in the Hello message. Each bridge compares the root identifier in a received Hello message with its own bridge ID and if the value of the received ID is smaller than its own ID, the bridge stops the transmission of the Hello message. In this way, as an end result, only the bridge having the smallest ID maintains the transmission of the Hello message. This bridge is the root bridge of the spanning tree. The other bridges are divided into two groups, i.e. bridges relaying frames (active bridges) and bridges which don't relay frames (backup bridges), according to a particular algorithm, which forms a spanning tree.
Also, in the case where some hindrances have occurred in transmission paths within a spanning tree once constructed, broadcast communication of the Hello message is effected and the spanning tree is reconstructed using the message. The root bridge is one of the active bridges. In this specification the active bridges other than the root bridge are called designated bridges.
However, according to the prior art techniques, no consideration was given to a network system having a hierarchical structure, in which a plurality of LANs having a relatively low speed (called sub-LANs or secondary LANs) are connected with an LAN having a high speed and a large capacity (called main LAN or primary LAN) and it has a problem results as follows.
That is, in a network according to the hierarchical routing method, a communication message or packet between 2 sub-LANs, which are in a positional relation in which they can communicate with each other by using a backup bridge, cannot help passing through the root bridge in the spanning tree and as a result there is a tendency for traffic to be concentrated in the neighborhood of the root bridge. It is desirable that this concentration of the traffic be dealt with by the main LAN having a high speed and a large capacity with respect to subLANs. However, according to the prior art techniques, since no consideration was given to providing network structure having a main LAN with a large data transmission capacity as described above, there was a possibility that a bridge connecting sub-LANs would serve as the root bridge in the spanning tree. In this case, there was the possibility that the root bridge will not be able to deal with the concentrated traffic with the result that it falls into an overcrowding state.